Field of the Invention
The present invention relates to imaging lenses built in camera modules using solid-state image sensors such as relatively compact low-profile CCD sensors and CMOS sensors which are mounted in mobile terminals such as mobile phones and smart phones or PDAs (Personal Digital Assistants).
Description of the Related Art
In recent years, there has been a trend that the cameras built in devices such as mobile terminals (mobile phones, smart phones, etc.) and PDAs provide high resolution to cope with an increase in the number of pixels. In addition, for increased convenience and better appearance of these devices, the demand for smaller low-profile camera modules mounted therein is growing. At the same time, imaging lenses built in camera modules are also strongly expected to provide high resolution, smallness (compactness), thinness (low-profileness) and high brightness (namely a small F-value) and also offer a wide field of view to capture an image of an object over a wide range.
In order to meet the recent trend toward an increase in the number of pixels, many types of imaging lenses composed of four or five constituent lenses (elements) have been proposed as imaging lenses built in the above-mentioned devices. However, an imaging lens composed of four constituent lenses can meet the demand for compactness because of the small number of constituent lenses but cannot provide a sufficient function to correct aberrations to achieve higher resolution. Also, an imaging lens composed of five constituent lenses can correct aberrations more easily and is more advantageous in providing higher resolution and higher performance but has difficulty in meeting the recent demand for compactness and a low-profile design.
In order to address the above problem, the present applicant has proposed an imaging lens described in JP-A-2013-15587 (Patent Document 1). The imaging lens includes four constituent lenses and has a diffraction optical surface on one lens surface to achieve high resolution and high performance without an increase in the number of constituent lenses.
Specifically, in Patent Document 1, the present applicant discloses an imaging lens which includes, from an object side, a biconvex first lens, a biconcave second lens, a meniscus third lens with positive refractive power having a concave surface on the object side, and a biconcave fourth lens, in which all the lens surfaces are aspheric, a diffraction optical surface with a chromatic dispersion function is formed on one of the object-side surface of the first lens to the object-side surface of the third lens, and all the constituent lenses are made of plastic material.
The imaging lens described in Patent Document 1 is a compact imaging lens which provides brightness with an F-value of about 2.4 and corrects chromatic aberrations and other aberrations properly. According to Patent Document 1, since the second lens has negative refractive power and is made of high-dispersion material and the diffraction optical surface is formed in an optimum place in the lens system, the imaging lens corrects axial and off-axial chromatic aberrations more properly and delivers higher imaging performance than the conventional imaging lenses composed of four constituent lenses. However, in order to meet the recent growing demand for imaging lenses which provide a wide field of view, high brightness, and compactness and high imaging performance, further improvement is necessary. In other words, the following problems must be addressed: one problem is that plastic material options are limited, and another problem is that high-dispersion materials generally have difficulty in correcting residual chromatic aberrations due to their partial dispersion characteristics as expressed by a quadratic curve. Whereas thanks to the diffraction optical surface formed on the optimum lens surface, chromatic aberrations can be corrected more properly than with a conventional refractive lens. However, it is difficult to improve the higher ability to correct particularly chromatic aberrations of magnification related to the above problem of the characteristics of the material and deliver higher imaging performance.